Embodiments disclosed herein relate to motor vehicle occupant compartment construction, and in particular to a school bus occupant rollover protective structure that reduces the risk of injuries in rollover accidents, simplifies construction, and reduces cost.
School bus rollovers are a type of vehicle accident, often resulting in injury, and efforts have been taken to provide improved rollover protection to the occupants, often schoolchildren. To this end, the United States Department of Transportation, through the National Highway Traffic and Safety Administration, has enacted several Federal Motor Vehicle Safety Standards (“FMVSS”) directed to the improved performance of school buses in a rollover.
One such Standard is FMVSS 220—School Bus Body Rollover (Roof Crush). This Standard applies a force equal to 1½ times the unloaded weight of the bus to the roof of the bus. The downward vertical movement of the force application plate is not to exceed 130 mm (5.125 inches). The length of the force application plate is 305 mm (12 inches) less than the length of the bus roof and 915 mm (36 inches) wide. Each emergency exit of the bus must operate in accordance with FMVSS 217 before, during, and after a roof crush test.
Another Standard is FMVSS 221—School Bus Body Joint Strength. In this Standard, each body panel joint shall hold the body panel to the member to which it is joined when subjected to a force of 60% of the tensile strength of the weakest joined body panel. A body panel is defined as a body component used on the interior or exterior surface to enclose the passenger compartment. The passenger compartment is defined as space within the school bus interior that is between a vertical transverse plane located 762 mm (30 inches) in front of the forward most passenger seating reference point and including a vertical transverse plane tangent to the rear interior wall of the bus.
Additional standards have been established by other organizations, such as the National School Transportation Association, and in particular its Specifications & Procedures—Side Intrusion. Under this standard, the bus body must withstand an intrusion force equal to the curb weight of the bus, but shall not exceed 88,960 N (20,000 pounds), whichever is less. The side of the bus is impacted at a location 559 to 610 mm (22 to 24 inches) above the floor line, with a 254 mm (10 inch) maximum diameter cylinder, 1,219 mm (48 inches) long in a horizontal plane. The cylinder must span two internal vertical structural members (bows). The cylinder shall not exceed 254 mm (10 inches) from its original point of contact. There can be no separation of lapped panels or construction joints.
Other standards include those of the State of Kentucky, Minimum Specifications for School Buses—Roof Intrusion. In this standard, there shall be no separation of a lapped panel joint when impacted at any point along the roof line on the outside surface, using an 203 mm (8 inch) diameter cylinder, 1,219 mm (48 inches) long, at a 30-45-degree angle, 25.4 to 76.3 mm (1 to 3 inches) above the top window line. The cylinder shall impact the roof line with the 1,219 mm (48 inch) dimension in a vertical plane with a force not to exceed 254 mm (10 inches) penetration into the passenger compartment.
The State of Colorado has adopted a Racking Test, in which a diagonal (racking) load test is applied to Type A, B, C, D school buses to assure adequate shear stiffness and strength of the bus body. With a force equal to 1½ times the Gross Vehicle Weight applied to the edge of the roof, the diagonal movement of the force at any point on the force application plate must not exceed 130 mm (5.125 inches), and each emergency exit shall operate in accordance with FMVSS 217 before, during and after the racking test. This loading is to be performed twice (two-cycle loading sequence). Seats may be installed in the test body in a manner that is identical to normal production.
Attempts to comply with these standards has resulted in the development of school bus fuselages with a multiplicity of generally U-shaped, one-piece rib members, spaced at intervals along the length of the body and coupled adjacent to their ends to the bus floor in an inverted position. These rib members were located between the interior and exterior panels and are joined to all of the panels. A plurality of stringer members, located along the joints between the exterior roof panels and the interior roof panels, define elongated roof stringers that extend substantially the entire length of the bus body and are secured to substantially every rib member. The rib members, stringer members, and elongated panels have been effective in increasing the safety of passengers in the bus body in the event of a collision or upset.
The problem with this current industry wide school bus design is that although most school bus fuselages pass these static tests, the tests are not necessarily predictive of the performance of the vehicle in actual dynamic rollover accidents. For example, in testing under FMVSS 220—School Bus Body Rollover (Roof Crush), the test performed by progressively loading the roof structure does not duplicate the multi-directional forces applied simultaneously to the roof structure in an actual dynamic rollover accident. In recent actual rollover accidents, the roof structure above the lower side glass sill has completely collapsed during the dynamic rollover event, even though the school bus design complied with FMVSS 220. Thus, it has been determined that current school bus designs should be improved to improve rollover protection to the occupants.